Vehicle manufacturers are today striving to increase engine efficiency and reduce fuel consumption. This is specifically an issue for manufacturers of heavy vehicles, such as trucks and buses. In vehicles with combustion engines some of the energy from the fuel is dissipated as heat through the exhaust pipes and the engine cooling system. By the use of a waste heat recovery system the heat from the exhaust gases may instead be used to produce mechanical work. The mechanical work may for example be transferred to the powertrain and thus be used to propel the vehicle. This way the engine efficiency and the fuel consumption may be improved.
Waste heat recovery systems are typically based on the Rankine cycle and thus comprise a working fluid, a pump for circulating the working fluid in a circuit, at least one evaporator, an expansion device and a condenser. The working fluid is suitably in a liquid state to start with. The pump pressurizes the working fluid which is pumped through the evaporator. The working fluid is heated by the exhaust gases lead through the evaporator and the working fluid thereby evaporates. The vapour is subsequently expanded in the expansion device. By means of the expansion device the recovered heat is converted into mechanical work. The vapour is thereafter cooled in the condenser, such that the working fluid is brought back to its initial liquid state. The condenser is thus typically connected to a cooling circuit, which could be part of the engine cooling system or a separate cooling system.
The operating temperature of such a waste heat recovery system is normally quite high and the thermal inertia of the system results in a high temperature even after the system has been shut down. Too high temperatures could damage the working fluid and other components of the waste heat recovery system. It is therefore important that the waste heat recovery system is cooled down before being shut down.
Document WO 2014/098843 A1 describes a method for operating a waste heat recovery system where a shutoff valve is arranged at the inlet side to the pump, such that the circulation of the working fluid may be stopped in the event an emergency shutdown is needed. Such emergency shutdown of the waste heat recovery system may be needed for example during engine braking, malfunction of the system or when the vehicle is stopped with the engine running for inspection or service.